With the ratchet-tooth lever escapement all of the impulse must be derived from the pallets, but in the club-tooth escapement we can divide the impulse planes between the pallets and the teeth to suit our fancy; or perhaps it would be better to say carry out theories, because we have it in our power, in this form of the lever escapement, to indulge ourselves in many changes of the relations of the several parts.  With the ratchet tooth the principal changes we could make would be from pallets with equidistant lockings to circular pallets.  The club-tooth escape wheel not only allows of circular pallets and equidistant lockings, but we can divide the impulse between the pallets and the teeth in such a way as will carry out many theoretical advantages which, after a full knowledge of the escapement action is acquired, will naturally suggest themselves.  In the escapement shown at Fig. 20 we have selected, as a very excellent example of this form of tooth, circular pallets of ten degrees fork action and ten and a half degrees of escape-wheel action.

It will be noticed that the pallets here are comparatively thin to those in general use; this condition is accomplished by deriving the principal part of the impulse from driving planes placed on the teeth.  As relates to the escape-wheel action of the ten and one-half degrees, which gives impulse to the escapement, five and one-half degrees are utilized by the driving planes on the teeth and five by the impulse face of the pallet.  Of the ten degrees of fork action, four and a half degrees relate to the impulse face of the teeth, one and a half degrees to lock, and four degrees to the driving plane of the pallets.

In delineating such a club-tooth escapement, we commence, as in former examples, by first assuming the center of the escape wheel at A, and with the dividers set at five inches sweeping the arc a a.  Through A we draw the vertical line A B’.  On the arc a a, and each side of its intersection with the line A B’, we lay off thirty degrees, as in former drawings, and through the points so established on the arc a a we draw the radial lines A b and A c.  From the intersection of the radial line A b with the arc a we draw the line h h at right angles to A b.  Where the line h intersects the radial lines A B’ is located the center of the pallet staff, as shown at B.  Inasmuch as we decided to let the pallet utilize five degrees of escape-wheel action, we take a space of two and a half degrees in the dividers, and on the arc a a lay off the said two and a half degrees to the left of this intersection, and through the point so established draw the radial line A g.  From B as a center we sweep the arc d d so it passes through the point of intersection of the arc a with the line A g.

[Illustration:  Fig. 20]